At a Glance

Why Get Tested?

To evaluate your red blood cells (RBCs), white blood cells (WBCs), and platelets, to distinguish between the different types of WBCs, and to determine their relative percentages in the blood; to help detect, diagnose, and/or monitor a range of deficiencies, diseases, and disorders involving blood cell production, function, and lifespan

Sample Required?

A blood sample drawn from a vein in your arm or by pricking a finger, ear or, in the case of an infant, a heel

Test Preparation Needed?

None

The Test Sample

What is being tested?

A blood smear is a drop of blood spread thinly onto a glass slide that is then treated with a special stain and examined under a microscope by a trained laboratorian. It is a snapshot of the cells that are present in the fluid portion of the blood (plasma) at the time the sample is obtained. The blood smear allows for the evaluation of these cells:

These cell populations are produced and mainly mature in the bone marrow and are eventually released into the bloodstream as needed. The number and type of each cell present in the blood is dynamic but is generally maintained by the body within specific ranges.

The drop of blood on the slide used for a blood smear contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets. Under the microscope, the stained WBCs can be easily seen and the number and type of cells present can be estimated. The laboratorian can compare their size, shape, and general appearance to the established appearance of "normal" cells. It is also possible to distinguish between the five different types of WBCs and to determine their relative percentages (manual differential). During this examination, the laboratorian can also evaluate the size, shape, and color (indicators of hemoglobin content) of the RBCs (RBC morphology) and also estimate the number of platelets present.

A variety of diseases and conditions can affect the number and appearance of blood cells. Examination of the blood smear can be used to support findings from other tests and examinations. For example, RBCs that appear larger and paler than normal may support other results that indicate a type of anemia. Similarly, the presence of WBCs that are not fully mature may add to information from other tests to help make a diagnosis of infection, malignancy, or other conditions.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or by pricking a finger, ear or, in the case of an infant, a heel.

Is any test preparation needed to ensure the quality of the sample?

The Test

How is it used?

A blood smear is often used as a follow-up test to abnormal results on a complete blood count (CBC) to evaluate the different types of blood cells. It may be used to help diagnose and/or monitor numerous conditions that affect blood cell populations.

For the test, a drop of blood is spread thinly onto a glass slide that is then treated with a special stain. Traditionally, trained laboratorians have examined blood smears manually using a microscope. More recently, automated digital systems have become available to help analyze blood smears more efficiently.

At one time, a blood smear was prepared on nearly everyone who had a CBC. With the development of more sophisticated, automated blood cell counting instruments, it has now become routine to provide an automated differential. However, if the results from an automated cell count and/or differential indicate the presence of abnormal white blood cells (WBCs), red blood cells (RBCs), and/or platelets or if there is reason to suspect that abnormal cells are present, then a blood smear will be performed.

A blood smear is often used to categorize and/or identify conditions that affect one or more type of blood cells and to monitor individuals undergoing treatment for these conditions. There are many diseases, disorders, and deficiencies that can affect the number and type of blood cells produced, their function, and their lifespan. Examples include anemia, myeloproliferative neoplasms, bone marrow disorders, and leukemia.

Usually, only normal, mature or nearly mature cells are released into the bloodstream, but certain circumstances can induce the bone marrow to release immature and/or abnormal cells into the circulation. When a significant number or type of abnormal cells are present, it can suggest a disease or condition and prompt a health practitioner to do further testing. Depending on the findings from a CBC and blood smear, follow-up testing may include tests such as:

When is it ordered?

The blood smear is primarily ordered to evaluate blood cells when a CBC with differential, performed with an automated blood cell counter, indicates the presence of abnormal or immature cells. It may also be performed when a person has signs and symptoms that suggest a condition affecting blood cell production or lifespan.

Examples of signs and symptoms that may indicate one of these blood disorders include:

What does the test result mean?

Findings from a blood smear evaluation are not always diagnostic in themselves and more often indicate the presence of an underlying condition, its severity, and the need for further diagnostic testing. The results are taken into consideration with the results of the CBC and other laboratory tests as well as the tested person's clinical signs and symptoms.

The results of a blood smear typically include a description of the appearance of the red blood cells, white blood cells, and platelets as well as any abnormalities that may be seen on the slide.

Red Blood Cells (RBCs) Normal, mature red blood cells are uniform in size (7-8 µm in diameter) and do not have a nucleus as most other cells do. They are round and flattened like a donut with a depression in the middle instead of a hole (biconcave). Due to the hemoglobin inside the RBCs, they appear pink to red in color with a pale center after staining the blood smear. When the appearance of RBCs (RBC morphology) is normal, it is often reported as normochromic and normocytic.

While not every RBC will be perfect, any significant number of cells that are different in shape or size may indicate the presence of disease. Some examples of conditions that can affect red blood cells include:

White Blood Cells (WBCs) As part of a blood smear evaluation, a manual WBC differential is performed. Typically, at least 100 WBCs are found, counted, and categorized according to type. The percentage of each type is calculated. In addition, the appearance (morphology) and stage of development of the WBCs are noted. White blood cells have a nucleus surrounded by cytoplasm. All WBCs are derived from bone marrow stem cells. In the bone marrow, they differentiate into two groups: granulocytic and lymphoid cells. They mature into five distinct types of WBCs.

Those with granules in their cytoplasm are also called granulocytes and include:

Neutrophils (10-18 µm) are cells that have cytoplasm with pink or purple granules. They compose the majority of WBCs in a healthy adult. They are involved in the defense against infections.

Eosinophils (10-15 µm) are easily recognized in stained smears with their large, red-orange granules. Generally low in number (1-3%), they most often increase in number in individuals with allergies and parasitic infections.

Basophils (10-15 µm) have large, black granules and are the least often seen type of WBC (1%).

The non-granulocytes include:

Monocytes are usually the largest of the WBCs (12-20 µm) and are often referred to as scavenger cells (phagocytes). They can ingest particles such as cellular debris, bacteria, or other insoluble particles.

Lymphocytes are smaller in size (10-12 µm) and have a small amount of cytoplasm and often a smooth, round nucleus. One type of lymphocyte, the B-cell, is responsible for the production of antibodies (immunoglobulins).

Numerous diseases and conditions can affect the absolute or relative number of WBCs and their appearance on a blood smear. Examples of some of the conditions include:

Bone marrow disorders — depending on the condition, may increase or decrease absolute and relative numbers of WBCs

Allergies — may affect the number of eosinophils

Leukemia or myelodysplastic or myeloproliferative neoplasm — immature white blood cells such as blasts may be seen on the blood smear; blasts are normally found in the bone marrow where WBCs are produced and mature before being released into the blood. If blasts are seen on a blood smear, they may indicate a serious bone marrow disease.

Platelets These are cell fragments that develop from large bone marrow cells called megakaryocytes. Upon release from the bone marrow, they appear as fragments in the peripheral blood. When there is blood vessel injury or other bleeding, the platelets become activated and begin to clump together to form aggregates, which is the beginning of a blood clot.

There must be a sufficient number of platelets to control bleeding. If there are too few, of if they don't function properly, the ability to form a clot becomes impaired and can be a life-threatening situation. In some people, too many platelets may be produced, which may result in interferences with the flow of blood, increasing a person's risk of developing a blood clot. These same people may also experience bleeding because many of the extra platelets may be dysfunctional even though they appear normal.

A platelet count is usually part of a CBC. An abnormally low number or high number of platelets may be further evaluated by preparing a blood smear to directly visualize any anomalies in shape or size. For example, large platelets or giant platelets may be seen in myeloproliferative neoplasms or immune thrombocytopenia, a condition in which the immune system inappropriately produces antibodies directed against platelets. (For more information, see the articles on Platelet Disorders and Excessive Clotting Disorders.)

Is there anything else I should know?

Findings on a blood smear that are abnormal are typically referred to a pathologist, often one with extensive experience in the study of blood (hematology), for further review and interpretation. Depending on the results, follow-up testing involving an examination of a bone marrow aspirate and biopsy may be required for a diagnosis.

Blood smears may be used to help diagnose malaria, a disease caused by a blood parasite. The parasite may be seen when a blood smear is examined under a microscope. Malaria rarely occurs in the U.S. and is usually only seen in travelers returning from areas where the parasite is more common (endemic). Therefore, the blood smear is not often used for this purpose in the U.S.

Some examples of situations or conditions that may affect or invalidate results of a blood smear include:

It has on a routine basis, but the automated blood cell counter usually evaluates the red blood cells (RBCs), white blood cells (WBCs), and platelets based on their shape, size, and electrical or photometric properties. There can be some variation in each cell type and numbers the body produces due to a variety of physiological and external stimuli. Use of an automated instrument can often identify the presence of abnormal cells but lacks the ability to definitively subclassify them. Cell fragments and platelet clumps, particularly if they are large in size, can be mistakenly counted as WBCs, thus falsely elevating a white cell count. A laboratorian can see these abnormalities on a blood smear and has been trained to identify and classify them appropriately.

Traditionally, blood smear review/interpretation has been done manually using a microscope, which is time-consuming and could be inconsistent among laboratorians. Automated digital morphology systems are now available to help analyze blood smears more efficiently.

2. If a drop of blood on a slide contains millions of RBCs, thousands of WBCs, and hundreds of thousands of platelets, how can the laboratorian see individual cells?

A specific technique is used to spread the drop of blood out across a glass slide into a "thumbprint" shape. Along the edges of this thumbprint the cells are only a single layer thick. This allows the cells in this area to be individually counted and evaluated under the microscope.

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Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

(2012 May 18). What is Anemia? National Heart Lung and Blood Institute [On-line information]. Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/anemia/ through http://www.nhlbi.nih.gov. Accessed July 2014.

Riley R, et.al. How to Prepare and Interpret Peripheral Blood Smear. Medical College of Virginia, Virginia Commonwealth University. Available online at http://www.pathology.vcu.edu/education/PathLab/pages/hematopath/pbs.html through http://www.pathology.vcu.edu. Accessed February 2011.

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This article was last reviewed on September 5, 2014. | This article was last modified on February 24, 2015.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.